1. Field of the Invention
This invention relates to a variable magnification lens system to be used on copying machines, and more particularly to a variable magnification duplicator lens system which is improved in copying performance quality over a wide magnification range.
2. Description of the Prior Art
The duplicator lens systems are generally required to meet the following conditions.
(1) For imaging a flat subject (an original) on a flat surface, the lens system is desired to have, along with uniform image resolution over the entire field area, good image surface characteristics holding the field curvature and astigmatism to the minimum;
(2) The aperture efficiency should be 100% to suppress drops in light amount in peripheral portions of the field area:
(3) The lens system should be compact and have a wide angle of field to meet the demands for compact and small-size copying machines;
(4) The distortion should be ignorably small;
(5) The contrast for CTF or modulation for MTF should be high enough over the entire variable magnification range, especially, in the low frequency range;
(6) The lens system should have a large aperture in terms of light passing power to permit high speed copying operations; and
(7) The system should be composed of a minimum number of lens elements of inexpensive glass material.
Considering that recent copying machine specifications generally include enlarged- and reduced-scale copying functions. In such a magnified duplication, the lens system should satisfy the above-stated requirements.
In order to avoid deteriorations in performance quality in reduced- and enlarged-scale copying operations, it has been known to change the copy scale without varying the distance from an original surface to a sensitive material surface as described, for example, in U.S. Pat. No. 3,865,470 using the so-called zoom lens unit. However, in consideration of the problems which are encountered in case of the zoom lens with a complicated scale varying mechanism, namely, the problems such as increase of the unit size, difficulties of making designs for high performances, increase in cost etc., it has been the general practice to use, instead of a zoom lens unit, a fixed focal point lens unit on ordinary copying machines for office use. In addition, where a fixed focal point lens is used, the so-called symmetrical lens system which has the lens elements arranged symmetrically relative to a stop is employed in most cases to prevent distortion and lateral chromatic aberrations in the position of the natural or equivalent scale which is regarded as a reference scale.
In a symmetrical lens system of this sort, the lens groups on the opposite sides of a stop are inverse to each other in refractive action. Accordingly, as clear from FIG. 13 which explains coma aberrations of a typical symmetrical lens system in equivalent scale position, the coma aberrations of the system as a whole can be corrected by making an adjustment such that the simple coma aberration 11 which is produced by overcorrection of a front lens group on the front side of a stop becomes substantially symmetric relative to the principal light ray P to offset same by the undercorrective coma aberration 12 which is produced by a rear lens group subtantially symmetrically relative to the principal light ray P.
In case of a fixed focal point lens unit, its position has to be shifted for a magnified duplication, i.e., toward the original surface (for duplication on an enlarged scale) and toward the sensitive material) for duplication on a reduced scale).
In reduced scale position, the upper light rays above the principal light ray pass through the front lens system close to its optical axis without undergoing strong refractive action of the front lens system, but as they pass through the rear lens system at a position away from its optical axis they undergo its strong converging action, producing undercorrective coma aberration. On the other hand, the lower light rays have a smaller angle of incidence to the front lens system than in the equivalent scale position, and therefore undergo the refraction by the lens system in a weakened degree, producing undercorrective coma aberration as shown in FIG. 14 and exhibiting considerable deteriorations especially in tangential coma aberration.
In enlarged scale position, the upper light rays above the principal ligh ray P of the rear lens system undergo weakly converging refractive action, changing from undercorrective coma aberration to somewhat overcorrective coma aberration. The lower light rays undergo strong converging action as the angle of incidence to the front lens system is increased as compared with that of the equivalent scale position, producing overcorrective coma aberration and as a whole resulting in overcorrective coma aberration as shown in FIG. 15.
Recently, copying machines are usually required to have a function of copy magnification over a broad range. When the range of magnification for copy scale reduction and enlargement is broadened, however, it becomes difficult to prevent deteriorations in lens performance quality. Especially, in case of a large-size copying machine intended for originals of 36 inches or greater in size, it becomes necessary to lengthen the focal length in order to widen the field angle in equivalent scale position to 50.degree. or greater for compactness of the machine. In the event of the focal length being increased in this way and adopting a design with a focal length set at 1.0 as a standard, even if the chromatic aberration in magnified scale position is of an ignorable degree in the standardized condition, it is likely that the aberration is multiplied according to the actual focal length in case of a full size unit of actual specifications, as a result of deteriorations in MTF of the unit.
Therefore, in order to correct the lateral chromatic aberration to a small amount which is practically ignorable even at a focal length as adopted in specifications, it has been necessary to use for the lens elements a costly glass material of extraordinary dispersion or to increase the number of lens elements, which is invariably reflected by increase in cost or in size of the lens system. Consequently, it is the general practice to prevent the deteriorations in lens performance in enlarging and reducing scale positions either by limiting the copy magnification to a narrow range where the performance quality is practically acceptable or by sacrificing the brightness or high light transmitting power of the lens or by restricting the angle of field.